Several different types of devices are available for the measurement of magnetic fields. Examples that operate at room temperature include flux gate magnetometers and magnetoresistors. A third family of devices is one of the oldest commercial field sensors, the Hall plate. All of these devices share a common trait; they measure a scalar value of magnetic field that represents a single component of the vector field to be measured. The family of Hall devices includes dozens, if not hundreds, of variations, for example, R. S. Popovic, “Hall-effect Devices,” Sens. Actuators 17, 39 (1989); and R. S. Popovic, “Hall Effect Devices”, (Adam Hilger, Bristol, 1991). One limitation of all of these Hall devices is that these devices only can measure a scalar field value proportional to the component of vector field that is perpendicular to the plane of the Hall plate.
In the art, one-dimensional arrays of Hall devices have been used to achieve a moderate magnetic field spatial resolution. A one-dimensional array of Hall crosses has been described by E. Zeldov et al., “Thermodynamic observation of first-order vortex-lattice melting transition in BiSrCaCuO,” Nature 375, 373 (1995). In this study, an array of ten Hall crosses, with dimensions of 3 μm by 3 μm for each sensor, was used to detect the motion of fluxons in a small sample of single crystal superconducting BiSrCaCuO. The sensors were fabricated on a GaAs/AlGaAs chip and a macroscopic (0.7 mm by 0.3 mm by 0.1 mm) sample was placed on top. Each Hall sensor had a DC sensitivity of order 0.1 Oe and each was able to sense the presence of a single fluxon when in proximity to the sensor. The spatial resolution of the measurement was therefore of the order of magnitude of the Hall sensor dimensions, a few microns.
One recent advancement in the art of measuring magnetic fields is provided by a hybrid Hall effect device. A hybrid Hall device is a simple bilayer magnetoelectronic device. The physical principles of operation are described in the publication “Hybrid Hall Effect Device,” by Mark Johnson, B. Bennett, M. J. Yang, M. M. Miller and B. V. Shanabrook, Appl. Phys. Lett. 71, (1997) and in U.S. Pat. No. 5,652,445 entitled “Hybrid Hall Effect Device and Method of Operating,” and are briefly described below with reference to FIGS. 1(a) and (b).
An example of one prior art hybrid Hall effect device is generally denoted 10 in FIGS. 1(a) and 1(b). A thin, microstructured ferromagnetic film F, denoted 11, is fabricated over a standard Hall cross 12 formed from a Hall plate 14, and positioned such that edge 13 of film 11 is disposed over the central region of the Hall cross 12. The film 11 is electrically isolated from the Hall cross 12, typically by a thin insulating layer 15.
The film 11 has a magnetization anisotropy in the film plane and acts as a local source of magnetic field. When an external magnetic field Hx{circumflex over (x)} causes the magnetization M of F to be along −{circumflex over (x)}, there is a positive magnetic pole density on the edge over the Hall cross 12, and a local negative field −|Bz| is generated in the vicinity of the carriers in the Hall plate 14 which comprises an InAs layer 16, a second insulating layer 17 and a substrate 18 (see FIG. 1(b)). The result is a positive sense voltage Vs=V+−V−. If the magnetization orientation is reversed, the sign of the pole density, the local field −|Bz|, and the sense voltage Vs is reversed. Thus, device 10 translates a horizontal magnetic field into a vertical magnetic field, thereby providing for the detection and measurement of the convolution of the {circumflex over (x)} and {circumflex over (z)} field components comprising the magnetic field.
When fabricated with appropriate magnetic properties, the film 11 has bistable magnetization and the resulting hybrid Hall effect device 10 has digital applications, such as nonvolatile storage. When fabricated with other magnetic properties, the magnetization of film 11 responds linearly with an external field. A hybrid Hall effect device engineered in this way can act as a sensor of in-plane magnetic fields.
One limitation with prior art hybrid Hall effect devices, such as device 10, is that these prior art devices cannot be used to determine the independent magnetic field vector components comprising a vector magnetic field, such as for determining the {circumflex over (x)} and the {circumflex over (z)} components of a magnetic field; device 10 merely measures the convolution of the {circumflex over (x)} and {circumflex over (z)} field components.